The present invention relates to a high pressure discharge lamp.
High pressure discharge lamps such as a high pressure sodium lamp or a metal halide lamp are widely used for exterior illumination of roads, public squares, sports facilities, etc. or in recent years for exterior illumination of commercial facilities or the like, based on advantageous features that they have a comparatively excellent color rendering property in addition to the merits of high efficiency and high luminance.
In order to light such high pressure discharge lamps, a starter generally is necessary. The starter is classified into two types: an external type incorporated into a lighting ballast and a lamp integrated type incorporated into a lamp itself. The latter lamp integrated type is in widespread use because by combining it with a simple copper iron reactance ballast, the cost of the lamp system is reduced.
Among the conventional built-in starter type high pressure discharge lamps there is one provided with a starter using a ferroelectric ceramic capacitor element with non-linear characteristics. This starter has the merit of high safety in practical use, and the startup performance also is comparatively excellent, so that it is more and more widely spread (See JP5(1992)-87940B, JP5(1992)-290985A).
FIG. 7 shows a conventional example of a built-in starter type high pressure sodium lamp. A starter of this lamp includes a series circuit of a ferroelectric ceramic capacitor (NCC) element 24 connected in parallel to an arc tube 23 of the high pressure sodium lamp and a bilateral semiconductor switching element 25. The starting operation is as follows.
When a power source 13 is applied, the NCC element 24 performs the operation of so-called current switching by cutting off the current based on its non-linear characteristics. Thereby, in a reactance ballast 14, a starting pulse voltage of 1500V to 2000V is induced for every half cycle in superposition on a source voltage, and with this voltage, the arc tube 23 is started. In this operation, the semiconductor switching element 25 serves to raise the starting pulse voltage even more by sharpening the current switching operation by the NCC element 24. In addition, in the configuration shown in FIG. 7, a start assisting conductor 28, which is connected in series to the NCC element 24 and the semiconductor switching element 25 via thermally-actuated switches 26, 27, is provided so as to be attached to the arc tube 23. Through this start assisting effect, the arc tube 23 can be started at a comparatively low starting pulse voltage. After the arc tube 23 has been started, the voltage applied to the NCC element 24 is reduced, and the current switching operation becomes impossible, so that the oscillation of the starting pulse voltage is stopped. Next, due to heat generation of the arc tube 23 after starting, the thermally-actuated switches 26, 27 made of bimetal elements are operated to be in an OFF state, and the steady lighting of the arc tube 23 is maintained in a state in which the starting circuit part including the NCC element 24 and the semiconductor switching element 25 are cut off from the lighting circuit of the arc tube.
According to the lamp configuration as a completed product, the arc tube 23 and all the starter parts excluding the semiconductor switching element 25 are mounted in an evacuated outer tube glass bulb 29. The semiconductor switching element 25 is positioned in a base for reducing its temperature. Therefore, for sealing the outer tube glass bulb, instead of an ordinary glass stem used for sealing two lead wires, a glass stem 17 used for sealing an outer tube glass bulb as shown in FIG. 8A, FIG. 8B is used. FIG. 8A is a plane view thereof, and FIG. 8B is the front view. In the glass stem 17, three lead wires 18, 19, 20 are sealed.
With regard to the lamp integrated with the starter using the NCC element, two problems related to safety were anticipated to arise during its life time. The first problem is that insulation deterioration of a ballast, a distribution cable, a base socket etc. arises in the case where the lamp becomes incapable of lighting and the starting pulse voltage is continued to be applied. It is dangerous for a human body to touch such a lighting device. The second problem is that in the case where a xenon gas for assisting a start, sodium or mercury filled inside the arc tube leaks from the outer tube glass bulb at the end of life and so on, an arc discharge is induced between the lead wires in the outer tube glass bulb due to the starting pulse voltage, and thus, an overcurrent flows due to this arc discharge. In this case, the ballast will be damaged by fire, or in some cases, the outer tube glass bulb will be broken.
In the starter according to the conventional technique shown in FIG. 7, in addition to the basic function of oscillating the starting pulse voltage, the following safety functions are added respectively to solve the two problems mentioned above.
(a) The ferroelectric property showing the non-linear characteristics of the NCC element 24 is maintained in a temperature range of not more than the so-called Curie temperature (normally, about 90xc2x0 C.). In a temperature range above this, it is changed to the paraelectric property and the non-linear characteristics disappear, and thus, the oscillation of the starting pulse voltage in FIG. 7 is stopped. In order to solve the first problem mentioned above by applying such temperature characteristics of the NCC element 24, a heating resistor 30 connected in parallel to the NCC element 24 and the semiconductor switching element 25 is positioned adjacent to the NCC element 24. Accordingly, even in the case where the arc tube 23 fails to light in spite of the oscillation of the starting pulse voltage, the temperature of the NCC element 24 rises quickly to the Curie temperature or higher by absorbing the heat from the heating resistor 30 in addition to the self heating of the NCC element 24 due to its operation, so that the oscillation of the starting pulse voltage is stopped in a relatively short time.
(b) To solve the second problem mentioned above, first of all, the NCC element 24 itself is designed and constructed to have the so-called self-destructive function. That is, when the starting pulse voltage is applied at the time when a xenon gas etc. leaks, a discharge breakdown occurs due to a creeping discharge between both electrode terminals and so forth, so that the NCC element 24 will be in a conducting state. In addition, a filament coil 31 is connected in series to the NCC element 24. The filament coil 31 has the so-called fuse function, that is, the filament coil 31 is fused by the flow of an excess current caused by the self-destruction and the conduction of the NCC element 24. In this way, by combining the self-destructive function of the NCC element 24 and the fuse function of the filament coil 31, the starter including the NCC element 24 is separated from the lighting circuit and becomes inoperative, so that the starting pulse oscillation is stopped. Even if a power source is applied again, the starter will never operate.
Furthermore, in the starter of FIG. 7, to conduct a stable control of the oscillation phase of the starter pulse voltage, a control resistor 32 is connected in parallel to the semiconductor switching element 25. When the NCC element 24 is used, due to the so-called depolarization at the time of transition from the ferroelectric property to he paraelectric property for every lighting of the arc tube 23, pyroelectricity flows in the NCC element 24. To prevent the non-linear characteristics of the NCC element 24 from deteriorating during the lamp life because of this pyroelectricity, a bypassing resistor for allowing the pyroelectricity to flow in a different way needs to be connected in parallel to the NCC element 24. In the circuit configuration of FIG. 7, the heating resistor 30 and the control resistor 32 function a such a bypass resistor for protection of the NCC element.
When the high pressure sodium lamp integrated with the conventional starter using the NCC element of the above-mentioned configuration is used actually in various applications, a new problem arose that the original starting function is deteriorated, and that in some cases, the lamp arc tube does not start surely, because of adding the above-mentioned safety functions.
In the conventional high pressure sodium lamp, as described above, in order to raise the temperature of the NCC element quickly to the Curie temperature to stop the oscillation of the starting pulse voltage when the lamp fails to light, a heating resistor is disposed adjacent to the NCC element. Even if the temperature of the NCC element is in a range lower than the Curie temperature, as the temperature thereof rises, the current switching operation becomes dull, and the starting pulse voltage to be induced is reduced. For example, at temperatures approximating the Curie temperature, the starting pulse voltage is reduced to xc2xd or less of the value at a normal temperature. On the other hand, when starting a high pressure discharge lamp, there inevitably is a so-called discharge starting lag time from the application of a power source to the starting of the lamp. In particular, in practical use, when the wiring distance from the ballast to the lamp installation position becomes long, and thus the damping of the starting pulse voltage becomes larger, the discharge starting lag time mentioned above becomes longer. In such a case where the discharge lag time is long, due to the quick temperature raise of the NCC element according to the effect of the heating resistor, the reduction of the starting pulse voltage becomes too large, so that the lamp arc tube cannot be started in some cases. This is the first problem.
As a second problem, it also became clear that when the lamp is at the end of its life, an arc discharge still arises in the outer tube glass bulb even though it is suppressed. This is due to the fact that according to the conventional technique, it takes a comparatively long time from the destruction and the conduction by the creeping discharge of the NCC element to the fusing of the filament coil for fuse 31, and that a variance range among the lamps also is comparatively large. For example, three are cases where it takes ten and several minutes at most until the fusing takes place. If it takes such a long time, there are cases where an arc discharge arises before the filament coil for fuse 31 is fused.
In addition to the two problems mentioned above, the following problems still remain unsolved. The problems are caused by the fact that the NCC element and the semiconductor switching element are mounted within the lamp which is to have a high temperature though they should avoid being operated at or exposed to a high temperature.
The first problem relates to restarting of the lamp after a steady lighting state. At the time of restarting, in order to induce a sufficient starting pulse voltage for starting the lamp, the NCC element needs to be operated at a relatively low temperature range of not more than about 65xc2x0 C. However, for example, when a lamp of a high watt 360W type is lit up and turned off inside an apparatus, the temperature of the NCC element is increased to 240xc2x0 C. or higher, and it takes a relatively long time to lower this temperature to the above temperature that is applicable to restarting of the lamp. Therefore, although the upper limit of he time for restarting a high pressure sodium lamp is set normally as 15 minutes, the actual restarting time of a high pressure sodium lamp needs to be set longer than that in some cases.
Another problem is the problem of characteristic deterioration of the semiconductor switching element 25 due to its exposure to a high temperature in a steady lighting state. Normally, the guaranteed heat-resistant temperature of the semiconductor switching element 25 at the time of storage (exposure) is defined as about 130xc2x0 C. However, even if the semiconductor switching element 25 is positioned inside the base to reduce its temperature as described above, in practical use, for example, when a high wattage lamp of the 360W type is lit inside an apparatus, the exposure temperature of the semiconductor switching element 25 substantially exceeds the specified value mentioned above.
Furthermore, since in this case the semiconductor switching element 25 is positioned inside the base by using the glass stem (See FIG. 8) as described above, and a distance between the lead wire connected to the semiconductor switching element 25 and the lead wire connected to another power source or ballast is short, both wires may contact each other or a discharge may be generated between both wires. To prevent this from occurring, a measure of coating the lead wires with an insulating tube is taken, but because of this, the manufacturing cost is increased.
As described above, in the starter using the NCC element according to the conventional technique, both the lamp starting function and the safety function still cannot be applied sufficiently to practical use. Furthermore, other various problems still remain to be solved, so that a further improvement of both functions and a solution to the various problems are desired by the market.
It is an object of the present invention to provide a high pressure discharge lamp integrated with a starter using a NCC element, having higher quality and safety, which is achieved by improving the lamp starting function an the safety function of the starter to a level that is sufficiently applicable to practical use.
A high pressure discharge lamp of the present invention comprises an arc tube; a starter including a ferroelectric ceramic capacitor element with non-linear characteristics and a semiconductor switching element in which the capacitor element and the switching element are connected in parallel to the arc tube; an outer tube glass bulb containing the arc tube and the starter; a glass stem for sealing the outer tube glass bulb; and a base positioned at an end portion of the outer tube glass bulb on the glass stem side. In the basic configuration of the present invention, a pulse stopping thermally-actuated switch is connected in series to the ferroelectric ceramic capacitor element and is operated to OFF due to heating by a heating resistor in a non-lighted state of the lamp.
According to this configuration, even when a discharge starting lag time is long, a temperature rise of the ferroelectric ceramic capacitor element is small, and a starting pulse voltage is maintained almost without any reduction. Therefore, a sure starting of the lamp can be obtained, and without accompanying a reduction of the starting function, the safety function against a non-lighted state of the lamp can be provided.
It is preferable that the high pressure discharge lamp has a starting circuit opening thermally-actuated switch for maintaining the starter in an OFF operation state at the time when the arc tube is lit, and that a recovery time of the pulse stopping thermally-actuated switch at the time of restarting the lamp is shorter than a recovery time of the starter circuit opening thermally-actuated switch. Thereby, a restarting of the lamp can be performed more surely.
Furthermore, it is preferable that the heating resistor is connected in parallel to the pulse stopping thermally-actuated switch and the ferroelectric ceramic capacitor element, and that a bypass resistor is connected in parallel to the pulse stopping thermally-actuated switch. Thereby, in the case where the pulse stopping thermally-actuated switch is off, the paraelectricity accompanied by the depolarization of the ferroelectric ceramic capacitor flows to the ferroelectric ceramic capacitor element via the heating resistor and the bypass resistor. Since the heating resistor also has the function of the bypass resistor of discharging the charge remaining in the ferroelectric ceramic capacitor, the starting circuit can be simplified.
Furthermore, it is preferable that the outer tube glass bulb is evacuated, and that in the outer tube glass bulb, a leaking filament coil connected in series to the ferroelectric ceramic capacitor and an electrode positioned adjacent to the leaking filament coil are provided so as to conduct an arc discharge between the coil and the electrode. Thereby, when a start assisting gas etc. leaks into the outer tube glass bulb at the end of the lamp life, the oscillation of the starting pulse voltage can be stopped more quickly, compared to the conventional lamp, and the generation of an arc discharge between lead wires in the outer tube glass bulb can be prevented more surely.
In the above-mentioned high pressure discharge lamp, it is preferable that a ceramic substrate is positioned between the arc tube and the glass stem in such a manner that the ceramic substrate is substantially perpendicular to a tube axis of the arc tube, and that on the glass stem side of the ceramic substrate, the ferroelectric ceramic capacitor element, the pulse stopping thermally-actuated switch and the heating resistor therefor, and a semiconductor switching element are positioned, and that on the arc tube side of the ceramic substrate, the starting circuit opening thermally-actuated switch is positioned.
Thereby, with regard to the problem in practical use of a high pressure discharge lamp equipped with a starter, the restarting of the lamp can be guaranteed and the restarting time can be reduced, and a rise of the exposure temperature of a semiconductor switching element in a steady lighting state of the lamp can be prevented.
In this high pressure discharge lamp, it is preferable that the pulse stopping thermally-actuated switch is positioned on the surface of the ceramic substrate on the glass stem side, and that a thickness of the ceramic substrate is set to be not more than 2.0 mm. Thereby, the recovery time of the pulse stopping thermally-actuated switch at the time of restarting the lamp is set easily to be shorter than the recovery time of the starting circuit opening thermally-actuated switch, sot hat a sure and normal lamp restarting can be performed.
In the above-mentioned high pressure discharge lamp, it is preferable that the pulse stopping thermally-actuated switch is positioned in parallel to the heating resistor, and that a resistance of the heating resistor is set in a range of 20 kxcexa9 to 40 kxcexa9, a power of the heating resistor is set in a range of 0.25W to 0.5W, and a distance between the pulse stopping thermally-actuated switch and the heating resistor is set to be not more than 2.0 mm. Thereby, even when the lamp fails to light in the condition of a low ambient temperature, the pulse stopping thermally-actuated switch is operated to OFF surely, and the oscillation of the starting pulse voltage can be stopped.
In this high pressure discharge lamp, it is preferable that a tip portion of the pulse stopping thermally-actuated switch is positioned in contact with the heating resistor. Thereby, even when the lamp fails to light in the condition of a low ambient temperature, the pulse stopping thermally-actuated switch is operated to OFF even more surely, and the oscillation of the starting pulse voltage can be stopped.
In the above-mentioned high pressure discharge lamp, it is preferable that the ferroelectric ceramic capacitor is placed substantially in parallel to the surface of the ceramic substrate on the glass stem side, and that a distance with the ceramic substrate is set to be not less than 0.5 mm. Thereby, the ferroelectric ceramic capacitor element can be prevented from breaking by the application of the starting pulse voltage.
In the above-mentioned high pressure discharge lamp, it is preferable that the semiconductor switching element is positioned outside the outer tube glass bulb and inside the base. Thereby, the exposure temperature in a steady lighting state of the semiconductor switching element positioned inside the base is reduced even more, compared to the one according to the conventional technique. As a result, also in a high watt type lamp, the exposure temperature can be suppressed substantially to the normal guaranteed heat-resistant temperature of not more than 130xc2x0 C., and the characteristic deterioration of the semiconductor switching element during the life of the lamp can be prevented.
In this high pressure discharge lamp, it is preferable that in the glass stem, one lead wire connected to one end of the semiconductor switching element and two lead wires connected to a power source are sealed, and that a sealing portion of the three lead wires in the glass stem has a cross section of a triangular shape, and that the three lead wires are sealed respectively in corners of the triangular shape. Thereby, the three lead wires are sealed with a comparatively long distance to each other, compared to the one according to the conventional technique, so that a contact of the lead wires or a discharge between the lead wires in the base can be prevented without covering an insulating tube.